Where are you getting this idea that Warriors are the "DPS tank" and Paladins are the "DEF tank"? And where does that put Dark Knights? Are they the "in the middle" tank?

The damage each of the tanks are able to do right now is pretty close, so there's no point in fussing over it.

Also, boosting Warrior damage even further would automatically make them the best tank and wouldn't be fair to the other two.